1. Field of the Invention
The invention relates to the detection of explosives in public locations and, more particularly, to a method and apparatus for detecting trace amounts of explosives on various surfaces.
Increasingly, public safety and the safety of government and military personnel depends on the detection of hidden explosives in venues such as airports and other transportation terminals, subway entrances, border crossings, sporting events, concerts, and other public gatherings. The presence of an explosive film or explosive particles on a surface is a possible indicator that larger amounts of explosives are nearby. For example, an explosive-laden fingerprint on a car's trunk lid, as may be detected by the present invention, may be an indicator that a large amount of explosives is inside the trunk.
The invention also relates to improvements in the eye-safety of laser emissions, so as to meet U.S. regulatory standards. Such improvements will allow for greater public use of laser-based instruments and techniques including, but not limited to, those disclosed herein for the detection of explosives.
2. Description of the Related Art
It is desirable to be able to detect explosives from a “safe distance.” A number of methods for such “stand-off” detection of explosives have been disclosed in the art, including certain methods utilizing lasers.
One laser-based method known in the art involves the use of Raman Spectroscopy, wherein a laser shining on a material excites molecular vibrations in the irradiated molecules. This results in emission of an optical spectrum that is unique to the chemical compounds present in the material. Variations on this method include Coherent Anti-Stokes Raman Spectroscopy (CARS), Surface Enhanced Raman Scattering (SERS), and Resonance Raman Spectroscopy.
Another method, known as Laser Induced Breakdown Spectroscopy (LIBS), uses a laser to ablate a small amount of a material and excite the ablated mass into a high-temperature plasma. The optical spectrum of the plasma then is used to identify the elemental composition of the ablated mass.
In Photodissociation/Laser Induced Fluorescence, laser light is used to photodissociate any unstable molecules on a surface and subsequently cause the dissociated molecular fragments to fluoresce. The fluorescence spectrum is then analyzed for the presence of telltale markers for fragments of explosive compounds.
A suite of related methods, collectively known as Laser Absorption Spectroscopy, is based on the absorption of laser light at different wavelengths as a laser beam passes through air above a surface containing explosives or explosive residues. Vapor from the explosive compounds absorbs the laser light at characteristic wavelengths that are unique to those compounds. This approach is also known by the acronym “LIDAR” (light detection and ranging), which is a general term often used to describe any kind of stand-off, laser-based vapor detection.
The phenomenon of laser absorption can also be used to detect the solid phase of explosive compounds. This technology exploits the fact that the reflectivity of a surface changes when a trace amount of explosive compound is present on it. The reflectivity is measured, usually in the mid-infrared region of the spectrum, over a range of wavelengths by using a tunable laser, most commonly a quantum cascade laser. As the laser wavelength changes, the magnitude of the reflected light is measured. The resulting spectrum of reflected light reflects the chemical makeup of the compound on the surface. Comparing the collected spectrum to a library of reflectance spectra collected from different explosives allows for identifying trace amounts of unknown compounds. See, e.g., U.S. Pat. Nos. 7,894,057 and 7,368,292.
The patent literature is replete with such methods of detection. For example, U.S. Pat. No. 5,420,905 discloses a method using resonance fluorescence and resonance absorption (preferably utilizing bremsstrahlung or other continuous-spectrum photon radiation) to detect explosives in a target such as a piece of luggage or other container. The utility of this method generally is limited to detecting explosives characterized by high concentrations of both nitrogen and oxygen, but also having a low concentration of carbon. The method utilizes a detecting apparatus to capture, measure, count, and record the energies of photons scattered from the target. This detecting apparatus requires appropriate filtering and shielding.
U.S. Pat. No. 5,818,047 discloses a method utilizing Raman Spectroscopy to detect Semtex plastic explosive, the active ingredients of which are RDX (cyclotrimethylene-trinitramine) and PETN (pentaerythritol-tetranitrate), in a sample such as a fingerprint on an aircraft boarding card.
U.S. Pat. No. 6,104,190 discloses a method and apparatus for detecting the presence of a nitramine explosive (such as RDX), wherein a radio frequency (RF) signal is emitted towards a target. If the target contains a chemical compound having a nitro group, excitation of such compound will produce a detectable Nuclear Quadrupole Resonance (NQR) signal.
U.S. Pat. No. 6,295,860 discloses an explosive detection system in which vapor leaking from luggage is sampled by a sampling probe; negative corona discharge is used to ionize the vapor; and a mass spectrometer is used to detect the ionized vapor, thereby determining whether or not an explosive is present.
U.S. Pat. No. 6,477,907 discloses an apparatus and method for detecting explosive-indicating compounds in subsurface soil. The apparatus has a probe with an adsorbent material on its surface and can be placed into soil beneath the ground surface, where the adsorbent material can adsorb explosive-indicating compounds. The explosive-indicating compounds are then desorbed and transferred as either a liquid or gas sample to a diagnostic tool (such as an ion-mobility spectrometer, a gas chromatograph, a high performance liquid chromatograph, a capillary electrophoresis chromatograph, a mass spectrometer, a Fourier-transform infrared spectrometer or a Raman spectrometer) for analysis.
U.S. Pat. No. 6,828,795 discloses an explosive detection system utilizing an ion mobility spectrometry instrument to detect the presence of trace molecules in air. A directed emission of photons, typically in the form of infrared or visible light, warms a target object, so as to significantly enhance vapor emission therefrom, which improves the sampling efficiency. A cyclone sampling nozzle also improves the sampling efficiency, particularly when the sampling needs to be performed at a distance from the air intake.
U.S. Pat. No. 6,928,131 discloses a method, utilizing X-rays, to detect an explosive in an object, such as inside a piece of luggage or mail. X-ray images of the object are used to detect areas containing a high density of organic materials and/or unidentified articles therein. Any such areas then are further characterized with respect to location, dimensions and supposed mass of any unidentified article therein. The method further includes thermal neutron irradiation of the area containing any such unidentified article; recording the output using gamma-ray detectors; determining a threshold value for the overall gamma-ray intensity based on the supposed mass of explosive being detected; and determining the presence of an explosive in the event the threshold value of overall gamma-ray intensity is exceeded. When checking small-size objects with this method, the neutron irradiation step is preceded by replacing the ambient air by a gaseous medium not containing nitrogen.
U.S. Pat. No. 6,967,103 discloses an explosives detector utilizing an array of molecularly imprinted polymer (MIP) coated, bifurcated fiber optic cables to form an image of a target molecule source. Individual sensor fiber assemblies, each with a calibrated airflow, are used to expose the fibers to the target molecule. The detector energizes a dedicated excitation light source for each fiber, while simultaneously reading and processing the intensity of the resulting fluorescence that is indicative of the concentration of the target molecule. Processing electronics precisely controls the excitation current, and measures the detected signal from a plurality of narrow band pass filters and photodiodes. A computer then processes the data to form and display an image of the target molecule source.
Finally, U.S. Pat. No. 7,239,974 discloses a method for monitoring thermal emissivity levels of human traffic in public venues. The method uses an infrared detector such as a quantum well infrared photodetector (QWIP) equipped camera. Based on differential emissivity calculations, a determination is made whether the monitored emissivity level corresponds to at least one calibrated emissivity level associated with an explosive material. The monitored emissivity levels are calibrated to eliminate the effects of other synthetic objects such as clothing, personal items, and other harmless objects. The monitored emissivity levels also are buffered against changes in environmental factors.
The above-described methods have multiple disadvantages. Typically, such methods involve optical emissions. Detection and analysis of these emissions may require very expensive equipment (e.g., a spectrometer). Moreover, such optical emissions may not be unique to explosives or may be difficult to discern from emissions caused by (a) the particular substrate on which an explosive film is deposited or (b) other compounds also deposited on the substrate (e.g., dirt, grease). Such methods also may require the use of multiple lasers, adding to the cost and complexity of the respective systems. Significantly, the efficacy of the above methods may be limited to detection of nitrogen-based explosives, so that other types of explosives are not identified. Finally, many of these methods are not sufficiently rapid, requiring many seconds or minutes to complete an analysis. Thus, when such prior art methods are used to test luggage or other personal items for explosives, unacceptable delays may arise, especially under circumstances where the respective owners of the luggage or other personal items are required to wait during the test.
Thus, it would be desirable to be able to provide a quicker, more accurate, and less expensive means of detecting a greater variety of explosives, especially in areas having a high volume of human traffic.